My vette, which has been running fairly well, has developed a hesitation and an occasional backfire, sometimes thru the carb, sometimes thru the pipes. I just installed an adaptor (spread to square bore) and that didn't help (no surprise there, just wanted to eliminate any vacuum leaks) double checked all plug wires at both ends, good connections. Hesitation is only at 1800 rpm and less, under gentle accelleration. At hiway speed she's smooth as silk, and she idles with a slight lope at 650 rpm. Timing was reset this evening at 12 btdc (was at 16 btdc) the balancer is new, no slippage yet. I'm gonna pull plugs this weekend. Could the valves need adjusting? I can't hear any noise above my exhaust (I have a small header leak, just installed new header gaskets, one side leaks slightly, but I'll get that sealed up this weekend, too) so I can't really tell. Power is good, just she's acting figit-ey in the lower rpm range. Weiand 8004 intake, edelbrock 1406 carb, CC XE262 stock smog heads, Summit headers into true duals with generic turbo mufflers, Accel coil and wires, running 93 octane fuel. She only pings if the temp guage gets up to 200, that's why I retarded the timing (she used to never ping at all) and she doesn't ping under WOT, just in a little area under part throttle acceleration and if I push just a little harder, ping goes away. No ping at cruise at all. I ran 89 octane (just to see if I could) and have the same symptoms. I hope that the new carb adaptor will help clear up some of the pinging, I think a vacuum leak at the carb base was leaning out the mixture and causing the ping (haven't tested that yet, I will tomorrow). The backfiring has me a little concerned. I plan on adjusting the valves and possibly re-visiting carb tuning to try and exorcise these demons. Recently I pulled the intake to inspect the gaskets and they were oil soaked. I installed new gaskets and the problem has lessened, but is still there (pushing oil thru the pcv, leaking around the plugs). I will also be installing higher quality gaskets this weekend, too (I'm gonna be a busy boy, but it'll be fun). So, since I'm gonna be under the hood all day Saturday anyway, is there anything else I need to check?

To recap:

Saturday I will:

Pull and inspect plugs,
Install new intake gaskets
Install new header gaskets
Install new valve cover gaskets
Adjust valve lash
Reset timing
Re-tune carb

So if y'all think of anything else, lemme know.

Thanks for any help y'all can give!

 

???? carbon build up breaking loose and hanging a valve once in a while

 

Good one! I'll do the water trick, too.

 

Techron seems to have done some good on a 305 that was bad, 4 bottle's in 3 tanks of fuel,,,if you know the water treatment do it, then 1 of the Techron do a filter change and oil,,, from the extra
carbon

 

Where can I get the Techron?

 

Ok, so the backfiring is due to a fouled plug. The other issues I'll deal with this weekend. Thanks for the help Comp!

 

How do the other plugs look? The bad intake seal will pull in oil and foul the plugs. Change the fouled plug, drive it for a while and check it again. See if it looks any better.

If it looks bad on all of them you might just need a hotter set of plugs. I had a little misfire in one of my rides a few months ago and when I stepped up the heat range, it went away.

-Mark.

 

I dont know much about the Edelbrock barbs but it sounds to me like you have a bad accelerator pump or transition circuit.

 

Contrail -
A few things to do:

First, don't retard the timing - this will make your heating problem worse and the hesitation worse. Set your timing up for 36 total advance; this will most likely put you at about 16-18 degrees advanced. You need this much advance with your setup. Then, run your vacuum advance off of a manifold vacuum source instead of the ported source. This will give you a bunch of advance and will reduce engine temp along with improving off-idle and part throttle response.

The Edelbrock carbs are jetted very lean, but tend to work okay with stock exhaust manifolds. With your headers you need to bump the jet size up about 2 sizes. This will take care of the stumble and further reduce the tendency for pinging, allowing the 36-degree total timing setting.